Printing methods wherein toner particles are used to form an image present several advantages. The printing method is a dry method--in case dry toner particles are used--and the toner particles, comprising a resin can be tailored so as to give weatherproof images, etc. Other printing methods, e.g. ink-jet printing, thermal printing, etc., show, when compared to toner printing, one or more deficiencies. Up until now, large format printing, using ink-jet is a well known technique, but the inks used in the printing, show often problems with weatherability, especially regarding the water-fastness. Therefore, in large format printing, e.g. poster printing, billboard printing, wherein the weatherability of the print is very important, silk-screen printing is still a dominant printing method. This method has however its own drawbacks. The method is rather time consuming since for every colour a dedicated screen has to be made and printed and the method is basically analogue.
More and more images to be printed are available in digital form, so that also in the printing of large formats, digital addressable printing techniques become indispensable. The use of toner based systems in such printing techniques is however not so straightforward. All toner depositing means available today, be it electrophotographic means or direct electrostatic means, are at least in one direction as large as the substrate to be printed. This means that in printing on substrates where the dimensions are measured in meter instead of in centimetre, as in, e.g., billboard printing, the toner depositing means must be very large (page-wide), since the printing speed equals intrinsically the speed of the digital printing process in the direction perpendicular to said page-wide array.
In direct electrostatic printing, this means that a very large array of printing apertures has to be provided, wherein each of the apertures has to be addressed separately. Moreover, the distance of said printing apertures to the substrate to be printed has to be controlled within tight limits, thus complicating the construction of a printer.
In electrophotography, ionography or magnetography, wherein an electrostatic or magnetic latent image is developed by dry toner particles and the developed image transferred to the substrate, a very wide latent image bearing member, without minute flaws that can deteriorate the image quality must be provided. Providing a large printer with such a latent image bearing member increases the costs of it to an almost unacceptable level. In case dry toning particles are used, the stable supply of toner particles to the development zone poses problems. The use of page wide applicators, e.g. based on magnetic brush assemblies is technically difficult to achieve, due to the small tolerances that are needed in the toning process.
Nevertheless, the use of the dry toning particles might have a very high value in large format printing, since they have a low ecological impact in comparison to ink-jet, liquid toning systems, and even the classical ink based systems as used in offset printing, gravure printing or screen printing.
The use of dry toner based imaging technology to create output with a size larger than the size of the imaging units (toner depositing means) themselves, in both dimensions, has only scarcely been addressed in the past. In the UK-patent application GB-A-2 106 453, there is disclosed an electrophotographic printer based on the use of a small printhead, comprising as well a photoconductive drum and basic stations such as a charging unit, an imaging unit, a toning unit and a cleaning unit, said printing head enabling the toning of plain, normal paper supported on a backing plate. Said device is described as an electrophotographic printer of the contactless type. It is described to be suitable for the creation of different, complex characters, whereas it is described to contain one line of characters as working width, leaving however open the possibility that more than 1, e.g. 2 lines can be printed in one single pass. Possible extension to colour is mentioned. Another reference to a dry toner based imaging method implicitly capable of creating larger size images can be found in the document "Shuttle print head, a TONERJET fact sheet" published by Array Printers AB, Sweden, (1995), TONERJET is a trade name of Array Printers. In this document an analogy is made between ink-jet printing and tonerjet printing. A process currently used in ink-jet printing, namely using the shuttling of the print head or print array is substituted by the shuttling of a toner jetting print head or print head array. The same mechanism is used, enabling the contactless projection of the toning material, in order to create images. This disclosure regarding direct electrostatic printing does not address the problem of keeping the distance between the printhead structure and the substrate within tight limits.
In the case of the electrostatic printing as disclosed in GB-A-2 106 453, the problem of image deterioration when the transfer of the toner particles from the latent image bearing member to the substrate is done in contact mode has not been addressed and moreover said GB application is concerned with providing an extremely small printing device.
In JP-A-61 146564 a serial printer is disclosed comprising pressure means to press a pressure receiver on a picture-bearing material for transferring and fixing the picture developed on the picture-bearing member to a recording material. The advantage of this design is that there is no external force necessary to move the recording paper.
In JP-A-61 152463 a serial electrophotographic printer is disclosed with a carriage including an image holding body, an electrification unit , an exposure unit, a toner and a developing unit. The carriage moves over an image receiving member that is larger than the carriage and band-like portions of the image are printed on the image receiving member and after the printing of several bands, the image is fixed. Also in DE-90 13 772 U such a type of serial printer has been disclosed. In such a serial printer, wherein the image is printed in band by a carriage moving over the image receiving member in a direction perpendicular to the direction of movement of the image receiving member through the printer, the image receiving member stops during the printing of a band and moves after the printing of a first band to the position wherein a second band is printed and so on. The fixing unit, being installed over the total width of the image receiving member (i.e. over the dimension of the image receiving member perpendicular to the movement of it), and operating continuously fixes stronger when the image receiving layer is stopped for printing a band than when the receiving layer moves. Thus the fixing does not proceed evenly which can result in a deterioration of the image quality.
Therefore, serial printer wherein the carriage includes also means for fixing the image have been disclosed. In, e.g., JP-A-61 145649 a serial electrophotographic printer is disclosed with a carriage including an image holding body, an electrification unit, an exposure unit, a developing unit, a toner and a developing unit and a fixing unit. Also JP-A-56 077167 discloses such a printer. In these printers, due to the different pressure exerted on the image receiving member by the image holding body (a photoconductive drum) and the fixing roller the image receiving member, the risk exist that the image receiving member becomes wrinkled, and--due to this wrinkling--that the transport of the receiving member does no longer proceed smoothly.
In U.S. Pat. No. 5,561,503 a serial printer with a carriage including an image holding body, an electrification unit, an exposure unit, a developing unit, a toner and a developing unit and a fixing unit is disclosed, wherein by a special design of the carriage and the bearing of he fixing unit, the problem of the wrinkling is avoided.
However, in a printer according to U.S. Pat. No. 5,561,503, the temperature of the image receiving substrate can be high, since fixing temperatures of toner particles are easily around 120.degree. C. and even around 150.degree. C. This heating of the substrate can result in dimensional changes of the substrate, e.g., an expansion or a shrinkage in the first band that is printed and whereon the image is fixed. When this occurs the exact registration of the band to be printed adjacent to the first band is difficult and even impossible. Even a limited mis-registration between the bands that are printed does deteriorate the image quality that can be achieved with the printer. This is especially so when full-colour prints have to be made.
Therefore further improvements, to make the techniques mentioned above suitable for printing with high image quality, both in small format printing and in large format printing are needed.